A microcrystal laser (micro-laser) is a laser having a solid-state gain-element in the form of a very thin crystal of a gain-medium, for example, neodymium-doped yttrium aluminum garnet (Nd:YAG) or neodymium-doped yttrium orthovandate (Nd:YVO4). A microcrystal laser can be of a monolithic form, in which a laser resonator (resonant cavity) is formed by applying reflective coatings (resonator minors) to opposite faces of the crystal. Here, the optical length of the resonator is determined by the thickness and the refractive index, of the crystal, and the temperature of the crystal.
A microcrystal laser can also have a so-called semi-monolithic form in which one resonator mirror is coated on one face of the crystal and the other resonator mirror is spaced apart from the crystal. This semi-monolithic form has an advantage over the monolithic form in that the resonator length can be selected independent of the thickness and material of the crystal. This semi-monolithic form also allows inclusion in the resonator of an active or passive Q-switch for providing pulsed operation of the laser.
Any laser-resonator has a number of resonant frequencies (wavelengths) determined by the optical length of the resonator. Any of these resonant wavelengths that fit within a gain-bandwidth of the gain-element can be lasing wavelengths (modes) of the laser resonator. The above referenced Nd:YAG and Nd:YVO4 gain media have a gain bandwidth of the order of about 1 nanometer (nm). In a “conventional” laser-resonator wherein the resonator length is a few centimeters or more, many modes will fit even within this 1 nm-bandwidth. Various relatively complex resonator arrangements are known for selecting one lasing mode from those possible lasing modes.
As the optical path length of a laser-resonator is reduced, the possible number of resonant wavelengths is reduced, and the wavelength-separation (free spectral range or FSR) of those resonant wavelengths within the gain-bandwidth of the gain-element is increased. In a micro-laser the resonator optical length is decreased until there are few enough resonant wavelengths within the gain-bandwidth of the gain-element (gain-crystal) that only one wavelength will be above a threshold gain-level, and, accordingly only that wavelength will “lase” (oscillate). The lasing efficiency, and accordingly the laser output power, will be determined, inter alia, by the wavelength of that lasing mode relative to the peak-gain wavelength in the gain-bandwidth. This lasing mode wavelength is dependent on the temperature of the micro-laser. It is taught that control of the temperature can provide for tuning of stability of the output wavelength of a micro-laser.
U.S. Pre-grant Publication No. 2011/0243158, assigned to the assignee of the present invention, and the complete disclosure of which is hereby incorporated by reference, describes a semi-monolithic micro-laser including a saturable semiconductor minor (SESAM). The SESAM is used as a resonator mirror and provides for passive Q-switched operation of the micro-laser.
FIG. 1 schematically depicts one practical disclosed structure 2 of the '158 micro-laser. The laser is assembled on a base 17. A thin gain-crystal 4 has a partially reflective and partially transmissive mirror 6 on one face thereof. The coated crystal is supported mirror-side-down on a transparent support 10. An anti-reflection coating 13 is provided on the opposite face of the crystal. A SESAM 8 is supported on base 17. The SESAM is spaced apart from the gain-crystal by spacers 16 and 16′ leaving an air gap 12 between the SESAM and the gain-crystal. A laser-resonator is formed between the SESAM and mirror 6. The optical length of the resonator provided by the optical thickness of the gain crystal and the thickness of the air is fine-tuned by thermal expansion or contraction of spacers 16 and 16′. While not disputing the practicality of the prior-art structure of FIG. 1, it has been determined that there is significant room for improvement, particularly with regard to providing tuning and temperature stability of the lasing wavelength.